How can I tell if a Caster Wheel is made of rubber or polyurethane?

I have a lot of wheels that I want to sell, but do not know if the wheels of rubber or polyurethane. Is there any evidence or touch the main visual difference / between the two materials that will help me determine what are those facts?

Rubber strain crystallizes. One of you put wheels under intense pressure, for example a press, then, if it crystallizes, which means they shatter, then you have rubber. Both materials are polymers and this is the only way I can think of giving the information I have. Hopefully, a expert will come and handle this.

Caster 3”. Swivel Plate 2-3/8×3-5/8. Rubber on Poly $3.52

Caster 4”. Swivel Plate 2-3/8×3-5/8. Rubber on Poly $3.70

Caster 5”. Swivel Plate 2-3/8×3-5/8. Rubber on Poly $3.98

Caster 3”. Rigid Plate 2-3/8×3-5/8. Rubber on Poly $2.79

Caster 4”. Rigid Plate 2-3/8×3-5/8. Rubber on Poly $3.12

Caster 5”. Rigid Plate 2-3/8×3-5/8. Rubber on Poly $3.41

Caster 3″. Swivel Plate 2-3/8×3-5/8. Rubber Poly Wheel $5.72

Metro 5MB 5″ caster swivel wheel with brake resilient rubber Set of 4 $59.99

Caster 4″. Swivel Plate 2-3/8×3-5/8. Rubber on Poly Whl $5.90

Caster 5″. Swivel Plate 2-3/8×3-5/8. Rubber Poly Wheel $6.49

4 Pcs 2″Ø Rubber Swivel Caster Wheels #22 $11.69

Caster 3″. Rigid Plate 2-3/8×3-5/8. Rubber Poly Wheel $4.20

Caster 4″. Rigid Plate 2-3/8×3-5/8. Rubber Poly Wheel $4.88

Caster 5″. Rigid Plate 2-3/8×3-5/8. Rubber Poly Wheel $4.93

4 Pcs 3″ Rubber Swivel Caster Wheels #14 $17.99

24 OFFICE CHAIR RUBBER CASTER SOFT WHEEL LAMINATE FLOOR $75.00

4 Pcs 2.5″ Rubber Swivel Caster Wheels #13 $15.29

150x 45mm Nonslip Rubber Industrial Trolley Caster Wheel $34.39

4″ Diameter Black Nonslip Rubber Wheel Silver Tone Metal Plate Fixed Caster $24.18

Home DIY 3 Inch Rubber Wheel Top Plate Mounted Caster $9.47

200 x 40mm Rubber Caster Wheel Red Black for Industrial Machines $44.02

Caster 5″. Swivel Plate 3-3/4×4-1/2. Rubber on Poly $6.80

Caster 5″. Swivel Plate 3-1/2×3-1/2.Rubber on Poly $6.80

Caster 3″. Total Lock Swivel Plate. Rubber Wheel $6.80

5″ Black Rubber Wheel Swivel Roller Caster for Trolley $31.41

Caster 4″. Total Lock Swivel Plate. Rubber Wheel $7.94

Caster 5″. Total Lock Swivel Plate. Rubber Wheel $8.47

Replacement 2″ Dia Gray Rubber Wheel Rigid Fixed Industrial Caster $6.47

Caster 6″. Swivel Plate 4×4-1/2. Rubber on Steel Wheel $12.94

Caster 3″ .Threaded Stem 3/8-16×1-1/2.TP Rubber $3.30

Caster 4″ .Threaded Stem 3/8-16×1-1/2.TP Rubber $3.56

Caster 5″ .Threaded Stem 3/8-16×1-1/2.TP Rubber $3.85

Caster 3″ .Threaded Stem 1/2-13×1-1/2.TP Rubber $3.30

Faultless 3″ Gray Rubber Grip Stem Caster with Brake! $4.14

Caster 4″ .Threaded Stem 1/2-13×1-1/2.TP Rubber $3.56

Caster 5″ .Threaded Stem 1/2-13×1-1/2.TP Rubber $3.85

Caster 3″ .Threaded Stem 1/2-13×1. Rubber on Poly Wheel $5.63

Caster 4″. Threaded Stem 1/2-13×1. Rubber on Poly Wheel $5.92

Caster 5″ .Threaded Stem 1/2-13×1. Rubber on Poly Wheel $6.49

Caster 3″. Threaded Stem 1/2-13×1-1/2. Rubber Wheel $5.63

Caster 4″. Threaded Stem 1/2-13×1-1/2. Rubber Wheel $5.92

Caster 5″. Threaded Stem 1/2-13×1-1/2. Rubber Wheel $6.49

Caster 3″. Threaded Stem 5/8-11×1-3/4. Rubber Wheel $5.63

Caster 4″. Threaded Stem 5/8-11×1-3/4. Rubber Wheel $5.92

1 Hard BLack Rubber Colson Replacement Caster 5″ wheel ROLLER CASTER TIRE $8.00

Caster 5″. Threaded Stem 5/8-11×1-3/4. Rubber Wheel $6.49

Caster 3″. Threaded Stem 5/8-18×1-3/4. Rubber Wheel $5.63

Caster 4″. Threaded Stem 5/8-18×1-3/4. Rubber Wheel $5.92

1 Hard BLack Rubber Colson Replacement Caster 5″wheel ROLLER TIRE CHAIR $8.00

Caster 5″. Threaded Stem 5/8-18×1-3/4. Rubber Wheel $6.49

Caster 3″. Threaded Stem 3/4-10×1-3/4. Rubber Wheel $5.63

Caster 4″. Threaded Stem 3/4-10×1-3/4. Rubber Wheel $4.91

Caster 5″. Threaded Stem 3/4-10×1-3/4. Rubber Wheel $6.49

Caster 3″. Total Lock Threaded Stem. Rubber Wheel $6.73

Caster 4″. Total Lock Threaded Stem. Rubber Wheel $9.24

Inter Metro Optional Black Zinc-plated Fork 3 inch Rubber Caster with Brake $13.24 The creative business center that lets you organize the workplace in style….

Inter Metro Optional Black Zinc-Plated Fork 3 inch Black Rubber Caster $43.99 The creative business center that lets you organize the workplace in style….

Bamboo Kitchen Island Cart with Wood & Black Granite Top $444.56 Expand your kitchen workspace with this natural finish work island featuring an elegant granite top inlay. The cart is complete with chrome finished hardware, towel rack two fixed shelves, one fixed shelf behind doors, two spacious pull out storage drawers, and heavy duty locking rubber casters for easy mobility and safety. The back of the cart is fully finished….

Rubbermaid 5E28 Deluxe Tool Tower Rack with Casters, Holds 40 Tools $48.00 Rubbermaid 5E2800MICHR Deluxe Tool Tower Rubbermaid 5E2800MICHR Deluxe Tool Tower Features: Tool tower Ideal for long handled garden tools Stores up to 40 tools Front clips and new curved tubing allows for head down storage and additional stability Molded in weed trimmer and electric cord holder Castors for easy mobility Easy to assemble No tools required Will not rust, dent, rot or peel 18″ x 37″…

Slipstick CB840 Large Castor Cup Gripper, Caramel $8.46 The patented Large Castor Cup is designed for castor wheels. It contains an O-ring specially formulated to protect floors and stop furniture on wheels from moving….